Unable to link 2 nodes to mixshader at once (python)
When a node has two inputs with the same name, for instance 2 color inputs, then you should pass the index instead of the input name.

# both inputs are called 'Color', so it's not enough
links.new(image_texture.outputs['Color'], emission_.inputs[0])
links.new(image_texture_2.outputs['Color'], emission_.inputs[1])


The shader mix node has 3 inputs, two are called 'Shader'. You'll need to use indices to distinguish each. 

# The mix.inputs[0] is the factor, indexing starts at zero
links.new(emission_.outputs['Emission'], mix.inputs[1])   # shader 1
links.new(trans.outputs['BSDF'], mix.inputs[2])           # shader 2